1. Field of the Invention
The present invention relates to a data reading method, a recording medium used in the method and a storage apparatus (information recording and reproducing device) using the method. In particular, the present invention relates to a data reading method when tracking control is performed using servo information, a recording medium used in the method and a storage apparatus (information recording and reproducing device) using the method.
In a magnetic recording and reproducing device such as a hard disk device, reading of data written in a recording medium should be guaranteed. However, there is a possibility that data written in a recording medium is not properly read due to degradation of the characteristics of the recording medium, adhesion of dirt to the recording medium, damage on the recording medium due to an external shock, external electrical noises or the like.
Commonly, in order to prevent such a situation that written data cannot be properly read, redundant data called an error correction code is added to data. When data is read, error detection and error correction are performed. Thereby, normally, original data can be restored to a certain extent.
Recently, as a recording device has been miniaturized and the storage capacity thereof has been increased, the S/N ratio of a read signal has decreased. Therefore, a restoration function performing data correction using the error correction code has been necessarily enhanced.
For example, in a disk device in about 1985, an ECC (Error Checking Code) for correcting an 11-bit burst error was used; in a disk device in about 1990, an ECC for correcting a 20-bit burst error was used; and in a disk device in about 1995, an ECC for correcting a more-than-40-bit burst error was being used. However, as a side effect of the enhancement of the error correction ability, even information which should not be changed may be erroneously changed by the error correction function.
An object of the present invention is to provide a method for preventing such an erroneous restoration when data is read.
2. Description of the Related Art
FIG. 1 shows a block diagram of an example of a hard disk drive in the related art. FIG. 2A shows a plan view of the hard disk drive after partially removing a top plate thereof. FIG. 2B shows an elevational sectional view of the hard disk drive.
The hard disk drive 1 is connected to a host apparatus 21, and stores, for example, a data processing result of the host apparatus 21. The hard disk drive 1 includes hard disks 2, a spindle motor 3 which rotates the hard disks 2 in the arrow A direction, magnetic heads 4 which perform information recording on the hard disks 2 and information reproduction from the hard disks 2, an arm 5 which holds the magnetic heads 4, and an actuator 7 which rotates the arm 5 in the arrow B directions about a rotation shaft 6 so as to move the magnetic heads 4 in radial directions of the hard disks 2. (Actually, as shown in FIG. 2B, the hard disk drive 1 includes the plurality of hard disks 2 and the plurality of magnetic heads 4. However, for the sake of simplification, FIG. 1 shows only one hard disk 2 and one magnetic head 4.) The hard disk drive 1 further includes a read/write control portion 8 which supplies recording signals to and receives reproduced signals from the magnetic heads 4, a formatter control portion 9 which is connected with the read/write control portion 8 and formats the reproduced signals into reproduced data and formats recording data into the recording signals, a data buffer 10 which temporarily stores the recording data and reproduced data, a data buffer control portion 11 which controls storage of the recording data and reproduced data in the data buffer 10, an I/F control portion 12 which acts as an interface between the host apparatus 21 and the hard disk drive 1, and a servo control portion 13 which performs servo control of the spindle motor 3 and the actuator 7. The hard disk drive 1 further includes an MPU (Micro Processing Unit) 14, a ROM 15 and a RAM 16 which are used for controlling the total operation of the hard disk drive 1
Data to be recorded on and data reproduced from the hard disks 2 are transmitted from and received by the host apparatus 21 via the magnetic heads 4, read/write control portion 8, formatter control portion 9, data buffer control portion 11, data buffer 10, and I/F control portion 12.
On each of the hard disks 2, a plurality of tracks are formed concentrically. Information is recorded on the tracks and recorded information is reproduced from the tracks. Each track is divided into a plurality of frames.
FIGS. 3A and 3B show an example of the arrangement of the hard disk 2 of the hard disk drive 1 in the related art.
As shown in FIG. 3A, the plurality of tracks Tr1 through Trn are formed on each hard disk 2. Each track is divided into a plurality of frames S1 through Sm.
The plurality of frames S1 through Sm are formed as a result of dividing each hard disk 2 by every predetermined angle. As shown in FIG. 3B, each frame includes a servo information area SB and a data area D.
In each servo information area SB, servo information is recorded. The servo information is used for controlling the position of each magnetic head 4 so that each magnetic head 4 can scan a desired track.
FIG. 4 shows an example of the data arrangement of the servo information recorded on the hard disks 2 of the hard disk drive 1 in the related art.
As shown in FIG. 4, the servo information area SB includes an AGC (Automatic Gain Control) area 17 in which an AGC signal for automatically adjusting the amplification factor for the reproduced signal read through the magnetic head 4 is recorded. The servo information area SB further includes an SM (Servo Mark) area 18 in which a servo mark SM indicating the start of information is recorded and a cylinder area 19 in which a cylinder address for identifying the track is recorded. The servo information area SB further includes a position area 20 in which track tracking information is recorded, which information is patterns for detecting an amount of displacement of the magnetic head 4 with respect to the track.
In the AGC area 17, an AGC signal, `1010 . . . 1010` is recorded for 90 clock pulses. In the SM area 18, the servo mark SM, `1000 . . . 00` is recorded for 18 clock pulses.
In the cylinder area 19, the cylinder address, `100X00X00 . . .` (each `X` is a value, `0` or `1`, in accordance with the cylinder address) of a so-called Gray code, is recorded for 99 clock pulses. Further, in the position area 20, the track tracking information, consisting of three patterns, each having a width approximately half the track width, is recorded for 240 clock pulses.
FIG. 5 shows an example of the data arrangement of the position area 20 of each track of the hard disks 2 of the hard disk drive 1 in the related art.
In the position area 20, a pattern A is recorded on the arrow-C-direction side of the cylinder-area-side of the track Trx-1, for the width half the track width for 80 clock pulses. Then, a pattern B is recorded on the arrow-D-direction side of the track Trx-1, for the width half the track width for 80 clock pulses. Then, a pattern C is recorded so that the center of the pattern C is the same as the center of the track Trx-1, for the width half the track width for 80 clock pulses.
The track tracking information, the patterns A, B and C, shown in FIG. 5, is recorded at the corresponding positions of each track of the hard disks 2.
In the hard disk drive 1, the servo information is read through the magnetic heads 4, and the servo control portion 13 controls the positions of the magnetic heads 4 so that the magnetic heads 4 scan desired tracks of the hard disks 2.
FIG. 6 shows an example of the operation flowchart of the servo control portion 13 of the hard disk drive 1 in the related art.
For a track of one of the hard disks 2, when the servo mark SM in the SM area 18 is detected (in a step S3-1), the cylinder address is read from the cylinder area 19 and the servo control portion 13 determines (in a step S3-2) whether or not the read cylinder address is the same as an expected address, that is, the address of desired data. When it is determined in the step S3-2 that the read cylinder address is the same as the address of the desired data, then the patterns A, B and C of the track tracking information are read from the position area 20, and the servo control portion 13 detects the amount of displacement of the magnetic head 4 with respect to the track, from the detected levels of the patterns A, B and C (in a step S3-3).
For example, when the detected levels of the patterns A, B and C, shown in FIG. 5, are the same as each other, it can be determined that the magnetic head 4 scans the desired track. When the detected level of the pattern B is low and the detected level of the pattern C is high, it can be determined that the magnetic head 4 is displaced in the arrow C direction. When the detected level of the pattern B is high and the detected level of the pattern A is low, it can be determined that the magnetic head 4 is displaced in the arrow D direction. In each case, the amount of the displacement is determined from the difference between the detected levels.
Thus, by calculating the differences between the detected levels of the patterns A, B and C, the amount of the displacement can be measured.
After the amount of the displacement is measured in the step S3-3, it is determined (in a step S3-4) whether or not the measured amount of the displacement exceeds a predetermined amount. When the measured amount of the displacement of the magnetic head 4 with respect to the track is smaller than a predetermined amount, for example, a certain amount shorter than the half of the track width, it is possible that the magnetic head 4 is returned to the desired track. Therefore, in a step S3-5, the actuator is driven by tracking control in accordance with the amount of the displacement measured in the step S3-3.
When the measured amount of the displacement of the magnetic head 4 with respect to the track is equal to or more than a predetermined amount, for example, an amount near to the half of the track width, it is not possible for the magnetic head 4 to be returned to the desired track, but the magnetic head 4 may be moved to the adjacent track, if the magnetic head 4 is moved by the tracking control in accordance with the measured amount of the displacement. Therefore, in such a case, the tracking control is not performed.
In the hard disk drive 1 in the related art, an ECC is additionally recorded in the hard disks 2, and, when data is read from the hard disk 2, the ECC is used for determining whether the reproduced data is correct. In the past, a cyclic code, such as a Fire code, which does not have a high error correction ability was used.
Recently, in order to increase a recording medium recording density, the interval between each pair of adjacent data recording tracks is narrowed, and also, the width of each data recording track is narrowed. Thereby, the output level of the signal read from the hard disk 2 is particularly feeble. As a result, the erroneous reading occurrence rate increases. Therefore, it is necessary to improve the ECC error correction ability. For this purpose, recently, a Reed-Solomon code which has a high error correction ability has been used.
However, by increasing the error correction ability, there is a possibility that error correction is performed inappropriately. For example, a case will now be considered where an external shock is applied to the hard disk drive 1 in which the servo information for causing the magnetic head 4 to track a desired track of the hard disk 2 was previously written in the hard disk 2.
By the external shock, the magnetic head 4 may move from the desired track to an adjacent track. In such a case, data of the adjacent track is erroneously read through the magnetic head 4. In such a case, when the magnetic head 4 moves from the desired track to the adjacent track while reading the cylinder address, the cylinder address cannot be adequately read and the ECC is used for obtaining the non-read portion of the cylinder address. When the error correction ability of the ECC is not high, it may not be possible to obtain the non-read portion of the cylinder address. If so, it is determined that the read data is not desired data. However, if the error correction ability of the ECC is high, the non-read portion of the cylinder address may be obtained. Thus, inappropriate error correction may be performed. If so, it is erroneously determined that the read data is the desired data even though the read data is not the data read from the desired track but the data read from the adjacent track.
Thus, there is a possibility that the data of the adjacent track is read and it is erroneously determined that the read data is the desired data. Such inappropriate error correction occurs because the method for detecting movement of the magnetic head between tracks has not been improved although the error correction ability has been improved.
In the related art, in the servo information, as shown in FIG. 4, the track tracking information is read from the position area 20 after the cylinder address is read from the cylinder area 19. By reading the track tracking information, the amount of displacement of the magnetic head 4 with respect to the track is determined as described above. Because Gray code is used for writing the cylinder address, the address of the desired track can be read even if the magnetic head 4 is located at a position near to the center between the desired track and the adjacent track (as shown in FIG. 7).
FIG. 7 illustrates a problem occurring in the hard disk drive in the related art.
As shown in FIG. 7, the position area 20 is scanned after the cylinder area 19 is scanned by the magnetic head 4. As indicated by the broken line in FIG. 7, the magnetic head 4 scans the left (desired) track first, and then, due to the external shock applied to the hard disk drive, the magnetic head passes the center between the two tracks and moves to the right (adjacent) track immediately before the magnetic head 4 completes reading of the cylinder address from the cylinder area 19. In such a case, even though the cylinder address has not been read completely, the ECC, having the high error correction ability, is used and thus, the complete cylinder address of the desired track is obtained. Thus, an inappropriate error correction is performed. As a result, it is determined that the read cylinder address is the same as the desired address. However, the magnetic head 4 reads the track tracking information of the right (adjacent) track. Because it is already determined that the read address is the same as the desired address, the servo control portion 13 performs tracking control so as to cause the magnetic head 4 to track the right (adjacent) track, by controlling the actuator 7, using the track tracking information read from the position area 20 of the right (adjacent) track. Then, the data is read from the data area D of the adjacent track. The data read from the adjacent track is erroneously determined to be the data read from the desired track because it has been already determined that the read address is the same as the desired address.
In the past, even when such a problematic situation occurred, because the cylinder address was additionally recorded in each portion of the data area D, which portion is used by the user as a unit for reading data from and writing data on the hard disk, it could be determined that the read data was not the data read from the desired track as a result of the cylinder address being recorded in each portion of the data area D being checked. However, recently, the cylinder address is not additionally recorded in each portion of the data area D, and the cylinder address is recorded only in the servo information area SB of each frame. Accordingly, the data read from the adjacent track may be erroneously determined to be the data read from the desired track.